With the increasing demand for automobile safety, automobile manufacturers have begun to equip passenger automobiles with air bags to enhance passenger safety. Air bags are devices that rapidly inflate with a gas when a detector on the automobile senses a collision. These passenger restraint systems are well-known in the art as described, for example, in U.S. Pat. No. 3,723,205 and U.S. Pat. No. 4,981,534, both by Scheffe, and incorporated herein by reference in their entirety. These devices should be designed with the highest degree of safety reasonably achievable to insure that the device will function properly at all times.
Inflation of the air bag can be accomplished by means of a gas stored under pressure, supplemented at the time of use by the addition of high-temperature combustion gas products produced by the burning of a gas-generating composition. In many instances, the inflation gases are produced solely by an ignited gas-generating composition.
It is important that the proper ignition of the gas-generating composition is reliable at the time of need. Also, it is important that these devices do not become inadvertently inflated when they are not needed.
A trigger device, commonly known as a header, is typically utilized to ignite a primer, or propellant, which in turn ignites the gas-generating composition. A header can include a conductive pin, surrounded by an insulative layer, that terminates on a thin bridge wire that traverses the insulative layer. When an electric current is passed to the conductive pin, the current passes to the bridge wire which rapidly heats due to its electrical resistance. This heat ignites the propellant, which subsequently ignites the gas-generating composition.
When compressed gas is used as an inflation gas, the heated bridge wire can ignite a primer which ruptures a compressed gas cylinder to allow the compressed gas to expand and inflate the air bag. Typically, the passenger side of an automobile uses the compressed gas inflation system.
The resistance and integrity of the bridge wire must be accurately controlled to assure proper and safe performance of the air bag device. Hence, the uniformity of the cross-sectional area and length of the bridge wire must be tightly controlled for accurate and reliable ignition of the propellant. Known headers suffer from many shortcomings in this respect.
To achieve acceptable uniformity and reliability, the conductive pin should be centered in the header within a true position tolerance of about 0.003 inch diameter (0.076 mm). That is, the true center of the pin should not deviate from the true center of the circumference of the header by more than about 0.0015 inch (0.038 mm). Proper centering of the conductive pin assures the proper resistance of the bridge wire. However, it is difficult to consistently achieve such reliably accurate tolerance levels in a large scale manufacturing environment.
Prior art headers typically utilize a glass composition formed from powdered glass for the insulative layer surrounding and sealing the conductive pin. One of the problems associated with using a powdered glass is that gas bubbles can easily form within the glass during the subsequent fusing operation.
A ceramic substrate is typically placed over the fused glass to provide the surface for depositing the bridge wire. However, there are many problems associated with using a ceramic substrate. For example, the substrate may be non-planar with regard to the surrounding metal surface. That is, the substrate may often sit higher or lower than the metal surface by, for example, about 0.0001 inch (0.0025 mm). This condition can cause the bridge wire to shear, particularly when the powdered propellant is compressed against the bridge wire during assembly. Therefore, any such headers must be rejected.
This problem is partly due to the fact that the surrounding metal surface is softer than the ceramic substrate and is removed at a higher rate during subsequent grinding operations.
Also, epoxy is utilized to hold the ceramic substrate in place. However, epoxy is prone to drying and becoming ineffective. Since these devices should provide a useful lifetime of at least about 15 years, epoxy is an unreliable method for holding the substrate in place. Further, the use of epoxy adds a costly manufacturing step.
It would be beneficial to have a process for producing these devices and similar devices that overcomes these problems. It would be beneficial if the conductive pin could be accurately centered within the feedthrough so that the length of the bridge wire is known and could be reproduced efficiently on a large scale. The centering of a conductive pin in a sealed insulator is also useful for other purposes, such as when producing hermetic coaxial connections. Further, it would be beneficial to minimize or possibly eliminate any bubbles within the glass that can cause uneven surface conditions. It would also be advantageous if the use of epoxy was eliminated to improve the long term reliability of the device. It would also be beneficial if the metal and the insulative substrate were machined to substantially the same level to minimize the chance of shearing the bridge wire due to a difference in the relative height of the insulative substrate and surrounding metal.